Water is an essential substance for all beings on earth to survive. However, wherever it is in the developed countries or in the developing countries, large quantities of pollutants have been discharged into water environment due to human activities and industrial production, causing a shortage of non-polluted water and thereby being a great challenge to public health and safety. Nowadays, each year in the world, there are 884 million people lacking safe drinking water and 1.8 million children died of water pollution related diseases. One of the essential problems to be solved for human development in the future decades is how to obtain safe drinking water. In addition, due to the increasing shortage of resources, another important theme for the future development of the world is energy conservation. However, the conventional water treatment processes require a large consumption of energy and resources. In US, 3˜20% of the annual consumption of energy and resources is used for water treatment processes. Therefore, there is a dilemma between the safety of drinking water and the consumption of energy and resources. CuiTently, researchers have developed numerous novel technologies and methods for drinking water treatment to increase the treatment efficiency, to minimize the energy consumption, to reduce the dosage of chemical agents and etc. Among those novel technologies and methods, membrane separation technology and adsorption technology have attracted most of the attention.
The application of membrane separation technology to water treatment has rapidly developed and there is a trend for the membrane technology becoming the core technology in the field of water treatment. Compared with water treatment technologies like sterilization, distillation and media filtration, membrane separation technology can separate pollutants in an efficient, selective and reliable way and without any need of chemical additives, heat energy or restoration. Currently, the membrane separation technology involves four processes, which are microfiltration, ultrafiltration (ultrafiltration), nanofiltration and reverse osmosis. Microfiltration can be used to remove suspended solids, protozoa and bacteria; ultrafiltration can effectively reject viruses and colloids; nanofiltration can achieve the removal of hardness, heavy metal and water-soluble organic substances; and reverse osmosis can be used to desalinize. Water pollutants have a wide size distribution, yet membrane separation technology can achieve an overall physical rejection of those pollutants. However, even though the nanofiltration technology and reverse osmosis technology can effectively remove small molecular water pollutants, the processes involved need to be driven by higher operating pressure and thereby are very energy-consuming. ultrafiltration process may replace the processes of turbidity removal and sterilization. ultrafiltration can reduce water turbidity under 0.1 NTU, resulting in the less or no consumption of coagulants. Pollution by coagulants can be avoided. Further, ultrafiltration can remove almost all of pathogenic microorganisms, thereby reducing the usage of disinfectants, and reducing the amount of disinfection by-products, so as to increase the chemical safety of water.
Adsorption technology is a promising method for removing small molecular water pollutants. It has advantages such as less energy-consuming, easy to operate and the extensive range of application. However, the current adsorption technology uses traditional adsorbents such as activated carbon, zeolites and natural fibers, which present several deficiencies such as low adsorptive capacity, poor adsorption selectivity and weak regeneration ability. Recently, novel nanoadsorbents such as nano-metal oxides, carbon nanotubes and porous graphene, have been attracted great attention of researchers and are given great expectations to overcome the deficiencies of traditional adsorbents. Compared with traditional adsorbents, nanoadsorbents have advantages such as high specific surface areas, short particle diffusion distances, more activated adsorption sites and easy to modify pore structure and surface properties, thereby able to obtain high adsorption capacity. It is easy to functionalize the surface of nanoadsorbents so as to make it selectively adsorptive. However, nanoadsorbents are often prepared in the form of fine powders which cause problems in separation/regeneration processes and potential safety concerns due to them leaching into water bodies. Therefore, the application of nanoadsorbents on the treatment of water is greatly restrained.
Recently, the spatiotemporal coupling technology, integrating multiple water treatment units into one device, has been garnered considerable attention. It can simplify the water treatment processes, increase the water treatment effectiveness, and overcome the deficiencies of single water treatment unit. The spatiotemporal coupling technology is particularly suitable for improving the current decentralized and point-of-use water treatment processes. Inspired by the above research, a strategy that can combine the advantages of nanoadsorbents and ultrafiltration membranes together and overcome their respective drawbacks in water decontamination, has been developed. Specifically, the nanoadsorbents were embedded in an ultrafiltration membrane matrix, forming a blend membrane with dual functions. Such technology has achieved simultaneous removal of small molecular, macromolecular and particulate pollutants under low pressure and overcome the deficiencies of ultrafiltration technology and nanoadsorption technology. However, it is generally accepted that the introduction of nanoadsorbents into polymeric casting solution will inevitably decrease the selectivity of the membranes and affect their performance, that is, the more nanoadsorbents blended in, the more probably the structure of ultrafiltration membrane would be damaged. In order to preserve the ultrafiltration performance, the content of nanoadsorbents in the membrane matrix was normally less, while the adsorption capacity is unsatisfactory. Another concern for blend membrane is that membrane matrix materials would encapsulate the nanoadsorbents blended therein, hence the less active adsorption sites. The deficiencies of traditional adsorption membranes have affected its practical application, therefore, it is of great significance to invent an adsorption ultrafiltration membrane with stable ultrafiltration property and enhanced adsorption capacity.